Announcements COMP 141. Accessing Characters Review. Using len function 10/30/2017. Strings II. Reminders: Program 6 - due 11/2

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1 Announcements COMP 141 Reminders: Program 6 - due 11/2 Strings II 1 2 # Prints 1 letter of city on each line city = Boston index = 0 while index < len(city): print(city[index]) index += 1 Using len function Accessing Characters Review Strings are stored character by character. Each character in a string is numbered by its position: C o m p u t e r The numbers shown here above the characters are called indices (singular: index) or positions. # Equivalent Code city = Boston for index in range(0, len(city)): print(city[index]) 3 4 1

2 Negative Indices String Indices Negative indexing can be used. Particularly useful for getting characters near the end of a string C o m p u t e r s[2] is the same as s[-6] both refer to m Two ways to use square brackets 1 number inside -> gives you 1 character of a string s[0] gives you the first character in s If s = Computer, s[0] gives you C 2 numbers inside (separated by a colon) -> gives you a substring or string slice To find last letter in string use: s[-1] 5 6 String Slicing Slice: span of items taken from a sequence, known as substring Slicing format: string[start : end] Expression will return a string containing a copy of the characters from start up to, but not including, end If start not specified, 0 is used for start index If end not specified, len(string) is used for end index Slicing expressions can include a step value and negative indexes relative to end of string String Slicing s[a:b] gives you a substring of s starting from index a and ending at index b C o m p u t e r s[0:1] -> C just like s[0] s[0:2] -> Co s[0:7] -> Compute s[3:6] -> put s[0:8] -> Computer 7 8 2

3 Indices Don t have to be Literal Numbers Say we have this code: s = input( Type in a string: ") x = int(len(s) / 2) print s[0:x]) What does this print? More Fun with Indices Examples using negative indices A negative index counts from the right side of the string, rather than from the left s = Computer print(s[-1]) print(s[-3:len(s)]) print(s[1:-1]) #prints r #prints ter #prints ompute 9 10 More Fun with Indices Slices don t need both left and right indices Missing left -> use 0 [far left of string] Missing right -> use len(s) [far right of string] Practice Write a function called total_seconds that takes one string argument. This argument will be a string of the form "M:SS" where M is a number of minutes (a single digit) and SS is a number of seconds (2 digits). This function should calculate the total number of seconds in this amount of time and return it as an integer. (Hint: Use string slicing/indices) s = Computer print(s[1:]) print(s[:5]) print(s[-2:]) #prints omputer #prints Compu #prints er Write a function called count_digits that returns the number of digits in a string. count_digits( abc123def5 ) returns 4 Write a function called sum_digits that returns the sum of all the digits in a string. sum_digits( abc123def5 ) returns 11 (because = 11)

4 String Concatenation Combines two strings into a new, longer string Uses the same plus sign as addition String Concatenation Unlike print(), string concatenation does not put spaces between your strings. s1 = CS141 s2 = rocks! bigstring = s1 + s2 print(bigstring) #prints CS141rocks! s1 = CS141 s2 = rocks! bigstring = s1 + + s2 print(bigstring) #prints CS141 rocks! Sample Problem Sample Problem Solution All professor addresses at Rhodes are constructed from the professor s last name, followed by the first initial of their first name. We want to design a function that take s a prof s first and last name and returns their address

5 The Repetition Operator Other String Methods Repetition operator: makes multiple copies of a string and joins them together The * symbol is a repetition operator when applied to a string and an integer String is left operand; number is right General format: string_to_copy * n Variable references a new string which contains multiple copies of the original string Programs commonly need to search for substrings Several methods to accomplish this: endswith(substring): checks if the string ends with substring Returns True or False startswith(substring): checks if the string starts with substring Returns True or False s = "a" s2 = s * 10 print(s2) #Output is aaaaaaaaaa More String Methods Using the find method Several methods to accomplish this (cont d): find(substring): searches for substring within the string Returns lowest index of the substring, or if the substring is not contained in the string, returns -1 replace(substring, new_string): Returns a copy of the string where every occurrence of substring is replaced with new_string Output: lastname_firstname_prg6.py

6 String Methods Testing, Searching, and Manipulating Strings You can use the in operator to determine whether one string is contained in another string General format: string1 in string2 string1 and string2 can be string literals or variables referencing strings Similarly you can use the not in operator to determine whether one string is not contained in another string Practice Write a function that returns a Rhodes student address. (Assume this address is for a new student). Your function will need to take in 4 arguments: first name, last name, middle name and class year. Write a function called reverse that takes a string argument and returns the string argument with all characters in the reverse order. reverse( Welsh ) returns hslew Write a function called filter_digits that returns only the digits from a string. filter_digits("abc123def5") returns "1235" Write a function called count_unique that counts the number of unique characters in a string. count_unique("abracadabra") returns 5. Write a function called count_dups that counts the number of back-to-back duplicated characters in a string. count_dups("balloon") returns